1.Field of the Invention
This invention relates to self-restoring fault isolation systems. More particularly, this invention relates to telephone tone amplifiers and tone distribution systems with combined fault isolation and self restoration to service of nonfaulted load circuits.
2. Description of the Prior Art
Most interest in fault isolation techniques is generated by two dominant concerns: a need for damage protection for the equipment associated with a faulty device, and the need to continue the operation of a main system regardless of the malfunctioning of a branch of the system.
Many different devices have been used to accomplish fault isolation in electrical systems, including the following: simple fuses, circuit breakers, relays (of both the normally open and normally closed types), and a variety of semiconductor circuits such as voltage clampers, and current limiting circuits.
The appropriateness of the use and placement of one or more of these devices depends upon the special features of the particular system under consideration; thus the possibilities fo creating ingenious combinations are myriad.
Telephone signal distribution apparatus are typical of systems that require both self protection and continuation of general service. For example, telephone central office equipment used to distribute the "call progress tones" (such as the dial tone, audible ring, and busy signal) is intended to serve many customers, and a defect in one or more of the customer loops should not be allowed to interfere with service to the remaining properly functioning loops.
The customer loops are comprised of a variety of possible loads that must be supplied with tone signals at various predetermined voltage levels. In order to provide output signals at these levels, it is necessary to have an output voltage level transformer with several voltage taps on its secondary winding. A difficulty with such an arrangement is that a fault ("short") occurring between the reference voltage ground level and any one of these taps will overload the transformer and decrease the voltage level at that tap as well as at the other taps. To avoid such an eventuality, each load is independently fused and one end of the secondary winding of the transformer is connected to a negative direct current (DC) potential source. This arrangement assures that the fuse in a faulty loop will be reliably and quickly blown by a large DC surge, thus isolating the defective load from the transformer.
The isolation technique described above is quite reliable, but it has the serious drawback of causing a high transient surge of reflected current to be generated in the primary of the transformer. This surge is transmitted through a coupling capacitor and into the output circuits of the power amplifier. This current surge is much higher than the maximum tone signal current that must be supplied by the power amplifier. Thus, the maximum current handling specification of the power amplifier is determined by the maximum value of the transient reflected current surge rather than by the maximum value of the amplifier's normal tone signal current. As a result, the power amplifier used in the prior art is necessarily a heavy duty amplifier which is rated for a power dissipation rating far above its normal operating output power. For example, in one embodiment of the prior art, a power amplifier rated at five watts is used to drive a one-watt load. These heavy duty amplifiers are expensive, bulky, discrete component-type circuits which are inconvenient to install and require specially trained technicians for maintenance and repair.
It is, therefore, an object of this invention to improve the efficiency, reliability, and maintainability of tone distribution circuits.
It is another object of this invention to provide a tone distribution circuit of decreased size, weight, and cost.